Add FrankenPHP builder variant

[jaydrogers]

👉 Describe the problem

Some users may want to replicate how serversideup/php builds FrankenPHP, especially if they want to add modules with XCaddy, etc.

👥 Problem evidence & reach

• Add frankenphp-builder variant #563

🏆 How to solve this problem

Create a "frankenphp-builder" image variant

This image would have all the build dependencies (similar to how FrankenPHP offers it).

🥰 Describe the "impact" on users?

• This could potentially make it easier for people to add their own extensions if needed

💯 How do we validate the problem is solved?

• FrankenPHP build process uses a serversideup/php:frankenphp-builder-* variation
• The GitHub Actions process has proper dependencies (to build the builder first, then use that to build FrankenPHP) and will fail if there is an error
• Other users can easily recompile or extend their FrankenPHP binary following our pre-built image